The present application relates to the electronics arts. It particularly relates to group III-nitride flip-chip bonded light emitting diodes for lighting applications, and will be described with specific reference thereto. However, it also finds application in conjunction with other types of flip-chip bonded epitaxial semiconductor devices, and the like.
In a flip-chip mounting configuration, a light emitting diode with a light-transmissive substrate and front-side electrodes is bonded “face down” to bonding bumps of a mount, that is, with the epitaxial layers proximate to the mount and a light-transmissive substrate distal from the mount. The flip-chip arrangement has a number of advantages, including improved thermal heat sinking due to the proximity of the active layers to the submount, and reduction of electrode shadowing losses.
In the flip-chip mounting configuration, light is extracted from the substrate side. For epitaxially grown light emitting diodes, the choices for substrate material can be highly restricted since the substrate is selected principally to provide a good base for the epitaxy. Thus, the substrate criteria include a narrow lattice constant range, a substantially atomically flat surface for nucleation of epitaxy, thermal stability at epitaxial growth temperatures, chemical compatibility with the epitaxial process, and so forth.
A problem can arise in the flip-chip configuration when the growth substrate is substantially light-absorbing over some or all of the spectral range of light emission. In this case, light extraction is reduced due to light absorption losses in the substrate. Moreover, even if a suitable optically transparent substrate is available, such as is the case for group III-nitride light emitting diodes which can be grown on transparent sapphire, optical reflective losses can occur at the interface between the substrate and the epitaxial layers due to an abrupt discontinuity in refractive index. Accordingly, it can be advantageous to remove the substrate to decrease optical losses.
However, the sapphire substrate serves as a lens which can improve the light extraction from the chip. Additionally, the GaN layer, which is exposed by the removal of the sapphire substrate, has a very smooth surface which results in a wave-guide effect. As a result, while the removal of the sapphire substrate from a nitride-based flip chip provides the above benefit, it also causes a significant degradation of the light output power of the LED chip.
It has been proposed to roughen the exposed layer of the GaN film to improve the light extraction. Some commonly used methods include dry etching the exposed layer such as reactive ion etching. However, dry etching has a possibility of damaging the chip and involves high costs. It also might etch and redeposit the underfill material that is typically used to support the GaN film after the substrate is removed. Another method commonly used is wet etching. The problem with wet etching is that GaN is not easily etched by most solutions and if it can be etched, the etch rate is too slow for effective processing. Another method used is photoelectrochemical etching which uses light and electronic bias to assist the etchant solution and speed up the wet etching. However, photoelectrochemical etching requires a complex setup to provide an electric potential over the surface being etched and deposit metal on GaN film that serves as an electrical contact. Furthermore, the LED chip must undergo additional processing both before and after the etching step.